main: "--- build done ---" delimiter on stderr for top-level #run

Tests that exercise top-level #run produce two interleaved
output streams: the interp's #run prints (flushed via
std.debug.print → stderr at core.zig:187/190) and the JIT-
executed main's prints (libc write fd=1 → stdout). When the
test runner captures both via 2>&1 the boundary between them
is invisible — the snapshot reads as one block.

Now `sx run` emits "--- build done ---\n" on stderr right
before invoking the JIT, when `hasTopLevelRun(root)` is true.
Tests without top-level #run keep their current snapshots
unchanged; only the 7 affected tests pick up the delimiter
between the build-time and run-time sections.

Example: 05-run flips from
    hello 25
    hello 25
to
    hello 25
    --- build done ---
    hello 25

— the first "hello 25" is from `#run main()` running at
compile time, the second is from JIT main() running at
runtime. The delimiter makes that explicit.

204/204 example tests + `zig build test` green.

tests/expected/05-run.txt

@@ -1,2 +1,3 @@
 hello 25
+--- build done ---
 hello 25

tests/expected/115-post-link-callback.txt

@@ -1 +1,2 @@
+--- build done ---
 runtime main

tests/expected/119-interp-cast-ptr-cmp.txt

@@ -1 +1,2 @@
+--- build done ---
 rt

tests/expected/120-interp-variadic-any.txt

@@ -1 +1,2 @@
+--- build done ---
 rt

tests/expected/121-ios-sim-bundle.txt

@@ -1 +1,2 @@
+--- build done ---
 ios-sim runtime main

tests/expected/38-build-config.txt

@@ -1,3 +1,4 @@
+--- build done ---
 build config: ok
 pointer size: 8
 os: macos

tests/expected/65-add-framework.txt

@@ -1 +1 @@
-
+--- build done ---